1. Field of Invention
The invention relates generally to optical filters and more particularly to an optical interference filter for transmitting a plurality of optical channels.
2. Description of Related Art
Optical interference filters rely on principles of interference that modify the intensities of the reflected light incident upon a surface. A familiar example of such interference is the colors created when light reflects from a thin layer of oil floating on water. Briefly stated, by modifying the interface of a substance and its environment with a third material, the reflectivity of the substance can be significantly altered. This principle is used in the fabrication of optical interference filters. These filters can be used as one of, or as the main filtering element in optical add/drop multiplexers employed in optical communication systems in order to select one or more channels from a transmission signal.
In its simplest form, an optical interference filter includes a cavity which is comprised of two partial reflectors (or mirrors) separated by a spacer. Each partial reflector, also referred to as a quarter-wave stack, is typically constructed by depositing alternating layers of high and low refractive index dielectric materials upon a substrate where each layer has an optical thickness of one quarter (or an odd integer multiple thereof) of the desired wavelength, λ0, of the filter, i.e. λ0/4. The optical thickness is defined as the physical thickness of the layer multiplied by the refractive index of the material. The spacer is typically a layer of low refractive index material (e.g., SiO2) having an optical thickness of λ0/2, or a multiple half-wave. Exemplary high and low refractive index dielectric materials are TiO2, Ta2O5 and SiO2, respectively. An interference filter has an associated transmission characteristic which is a function of the reflectance of the layers of high and low index materials associated with the stack.
In many applications, optical interference filters are constructed using multiple cavities. Typically, cavities are deposited on top of other cavities, with a quarter-wave layer of low index material therebetween. Multicavity filters produce transmission spectra that are preferred in optical communication systems where steep slopes and square passbands are needed to select or pass one or more wavelengths. In optical communications systems, a plurality of wavelengths may be used to carry a signal with each wavelength being referred to as an optical channel.
FIG. 1 illustrates an exemplary transmission spectrum for a mirror comprising a plurality of high/low refractive index dielectric layers. The mirror exhibits high reflectivity over a stopband centered at λ0 and rippled sidelobes including points A, B and C.
FIG. 2 is an exemplary transmission spectrum for a single cavity optical interference filter utilizing a pair of quarter-wave stacks each having the transmission spectrum shown in FIG. 1. As can be seen in FIG. 2 the transmission response is acceptable at wavelength λ0 (approximately 1550 nm). However, the response at wavelength λ1 (approximately 1310 nm) falls on the sidelobe and/or within the ripple band of the transmission spectrum, thereby making transmission of a particular wavelength in this range unreliable. Thus, transmission at a first wavelength λ0 may be reliable while transmission for wavelength λ1 within the ripple band or sidelobe slope are subject to variations in the transmission characteristic.
As noted above, optical systems can utilize one or more interference filters to select particular channels from a transmission signal. For example, a first filter may be used to select a payload channel associated with voice and/or data transmission in the 1.51 μm range and a second filter is used to select a service channel in the 1.3 μm or 1.6 μm range which carries system level and/or network monitoring information. The use of two separate filters, however, has several disadvantages. First, it increases overall system cost since it requires the manufacture and installation of two individual components. Secondly, optical networks typically have a predetermined loss budget, which, if exceeded, can compromise signal integrity. Each component, in this case an optical filter, contributes some loss to the overall network. By using two separate filters to select a payload channel and a service channel, each filter impacts a network's loss budget in a negative fashion.
The loss associated with individual filters is further compounded in wave division multiplexing (WDM) systems where the payload is delivered on a plurality of wavelengths and accompanied by a service channel on a separate wavelength. For example, in a six channel WDM system, seven filters are needed (six for payload and one for service channel) for both add and drop capability. This results in a total of fourteen filters greatly increasing loss to the network.
Thus, there is a need for a filtering element used with optical communication systems which is capable of selecting a plurality of optical passbands. There is a further need to provide such a filtering element which reliably selects at least one wavelength corresponding to a payload channel as well as a wavelength corresponding to a service channel within an optical network.